An electromagnetic-type fuel injector having a coil is well known as a fuel injector injecting fuel into a cylinder of an internal combustion engine. When the coil is energized, the fuel injector is opened to inject the fuel into the cylinder. A fuel injection controller drives such a fuel injector and controls the fuel injection to the internal combustion engine. Specifically, the fuel injection controller controls an energization start time at which an energization operation is started for energizing the coil. Further, the fuel injection controller controls a drive time period during which the energization operation has been conducted since the energization start time. Thereby, the fuel injection controller controls a fuel injection period and a fuel injection quantity.
Also, in this kind of fuel injection controller, a characteristic of a fuel injector is detected and the drive time period of the injector may be corrected according to the detected characteristic of the fuel injector.
JP-2010-532448A (EP-2174046A1) shows a method for detecting a characteristic of a fuel injector. In this method, an electric current flowing through the coil, which is decreasing from a starting time of valve-close period of an electromagnetic valve, is differentiated. The electromagnetic valve corresponds to a fuel injector and the starting time of valve-close period corresponds to an end time of the drive time period. Based on the derivative value of the electric current, a valve-close time of the injector is detected and a time period from the start time of the valve-close period until the valve-close time is computed as the characteristic of the fuel injector. Furthermore, based on the computed time period for valve-closing, a drive controlling duration, which corresponds to the drive time period, is computed so that a desired injection quantity is obtained.
Generally, in a fuel injection controller, in order to close a fuel injector immediately after the drive time period for the fuel injector is terminated, a counter-electromotive force generated by an energy accumulated in the coil is promptly consumed by the extinction, whereby the electric current flowing through a coil is rapidly decreased. The electric current flowing through the coil is referred to as an injector current.
For this reason, regarding such a fuel injection controller, when the method shown in JP-2010-532448A (EP-2174046A1) is applied to analyze a decreasing waveform of the injector current, it is likely that a sufficient detection accuracy may not be obtained in detecting the characteristic of the fuel injector because a decreasing period of the injector current is short. That is, a time length of the waveform of the injector current is short. The waveform of the injector current does not vary a lot according to a difference in characteristic of the fuel injector.
It is conceivable that an interval of an A/D conversion (analog-to-digital conversion) of the injector current is made shorter to improve the detection accuracy of the current waveform. However, in this method, an A-D converter of high-speed operation is necessary, which increases its cost.